A power supply to control the flow of energy between a first alternating current (AC) system and a second AC system is required in a variety of commercial and industrial applications, such as the control of AC motor operation. Some such power supplies convert the energy from a first frequency and voltage to a second frequency and voltage. One way to implement such a system is with one or more power cells that include two solid state converters with an intermediate DC link. A power cell is an electrical device that has a three-phase alternating current input and a single-phase alternating current output. One such system is described in U.S. Pat. No. 5,625,545 to Hammond, the disclosure of which is incorporated herein by reference in its entirety. Referring to FIG. 1 herein, Hammond discloses a power cell 110 that includes an AC-to-direct current (DC) converter 112—here, a three-phase diode-bridge rectifier—one or more DC capacitors 114, and a DC-to-AC converter 116—here, an H-bridge inverter. The rectifier 112 converts the input 118 AC voltage to a substantially constant DC voltage that is supported by the capacitors 114 that are connected across the rectifier 112 output. The output stage of the inverter 110 includes an H-bridge inverter 116 that includes two poles, a left pole and a right pole, each with two devices. The inverter 110 transforms the DC voltage across the DC capacitors 114 to an AC output 120 using pulse-width modulation (PWM) of the semiconductor devices in the H-bridge inverter 116. In some embodiments, the rectifier 112 may include three pairs of current-controlling devices, such as insulated gate bipolar transistors (IGBTs), diodes, switches or other devices. In some embodiments, the inverter 116 may contain two pairs of such devices.
A circuit including power cells such as 110 in FIG. 1, when connected to a load, such as a motor, can provide power from an input source to the motor when operating in the motoring mode. Such a power cell may sometimes be referred to as a unidirectional or two-quadrant (2Q) cell. However, when the motor speed needs to be reduced, power from the motor needs to be absorbed by the inverter. This mode of operation, when power must be absorbed by the inverter, is referred to as the regeneration mode. In such situations, some or all of the power cells are typically regenerative or four-quadrant (4Q) cells.
In the prior art, the ability to diagnose and predict failure of one or more cells has been limited. The disclosure contained herein describes attempts to solve this problem.